Microwave cooking often offers advantages of speed and convenience in heating foods. However, in the past, microwave cooking has been unsatisfactory for a number of food products. Because microwave cooking relies upon the dielectric heating of foods responsive to microwave radiation, the heating characteristics in a microwave oven for some food products is dramatically different from that experienced in a conventional oven. Problems with microwave cooking for a number of food products include the problem of undesirable temperature differentials. Oftentimes, food products cooked in a microwave oven will heat more in the interior due to dielectric heating caused by microwave radiation, than at the surface. This is directly contrary to the temperature differential achieved in a conventional oven, which is oftentimes desirable for foods which require a crisp surface or brown crust in order to have desirable taste characteristics. An additional problem with microwave cooking is that necessary temperatures for browning and crisping of the surface of food products have not been achieved. This is an old problem in the art, and many attempts have been made to solve it.
A related problem is the problem of moisture differentials which result in moisture migrating in an undesirable manner in a food product. Oftentimes, instead of evaporating or migrating from the surface of the food to the center of the food product, moisture will migrate the wrong way, i.e., from the center of the food product to the surface during microwave cooking. This has the effect of leaving the food surface soggy, which is oftentimes undesirable and detrimental to the texture and taste of the food.
Microwave cooking may also have a problem regarding the time characteristics of microwave cooking. For example, microwave cooking of cookies or bread may occur so rapidly that the cookie batter does not have time to spread properly, and the bread does not have time to rise properly.
In the past, attempts to solve some problems with microwave cooking have involved the use of susceptors which heat in response to microwave radiation. Typically, susceptors have been used which contain a thin film of metal, usually aluminum, deposited upon a substrate. Such susceptors typically have been characterized by surface resistivities in the range of 10 to 500 ohms per square. Such thin film susceptors have exhibited problems in the past related to the deterioration of such susceptors, when exposed to microwave radiation. Typically, susceptors deteriorate or break up, and become less reflective and more transmissive to microwave radiation as they are heated in a microwave oven. For many food products, this is undesirable. At present, there is no way known to applicants in which a practical and disposable susceptor can be manufactured economically which will not break up and significantly change its performance characteristics when exposed to microwave radiation. In the past, it has not been possible, to applicants' knowledge, to produce a susceptor with a particular combination of reflected, transmitted and absorbed power which would substantially remain at the same percentages of reflected, transmitted and absorbed power during microwave radiation. In the past, there has been no practical way of controlling deterioration of a susceptor during microwave irradiation.
Susceptors used in the past have also suffered from nonuniform heating. Thus, prior attempts to use susceptors to minimize some of the problems of microwave cooking have resulted in additional problems of food products which are overheated in some places and underheated in other places due to the nonuniform heating of the susceptor. For example, attempts to heat large pizzas have generally resulted in overheating of the outside of the pizza, and underheating of the center of the pizza.
In the past, there has been no practical way of controlling the rate of temperature rise of the susceptor. The only variables available to affect the temperature rise have been choosing the resistivity of the susceptor material, and the strength with which the metallized film is glued to its paper support. However, the surface resistance of the susceptor material changes during microwave irradiation, and the metal film breaks up. Thus, both variables available to affect the rate of temperature rise change during microwave heating.
From the above discussion it will be clear that prior art microwave cooking systems have been unsatisfactory in many respects.